1. Field of the Invention
The present invention relates to a process for forming a photoresist pattern using a photolithography process with a silylation step, and in particular to an improved process for forming a photoresist pattern that is optimized to form an ultrafine pattern. The preferred embodiment of the invention comprises employing a top surface imaging ("TSI") process, controlling process conditions such as temperature and time and employing a photoresist composition including cross-linking agents ("cross-linkers"). Preferably the cross-linker is a polymer of a cross-linking monomer represented by Chemical Formula 1 or 2 set forth below.
2. Description of the Background Art
The unique problems that may occur in forming an ultrafine pattern are pattern collapse in the developing step and deficiency of resolution. In addition, in the fabrication of a 4 G DRAM or 16 G DRAM semiconductor elements employing an ultrafine circuit below 100 nm, the photoresist layer should be thin in order to form a fine pattern. However, when the photoresist layer is thin, the etching process cannot be performed. Accordingly, the photoresist composition forming the layer must have etching resistance; however, it is difficult to enhance the etching resistance of a photoresist. Moreover, when the pattern is formed by employing DUV (Deep Ultra Violet) light, such as ArF and EUV, the pattern may be deformed by the energy of the optical system. As a result, there has been strong demand for a highly sensitive photoresist that can form a pattern at a weaker light energy level.
The only known process that overcomes the aforementioned disadvantages is a top surface imaging (TSI) process using silylation. However, even the TSI process does not make it possible to form an ultrafine pattern in the 4 G or 16 G DRAM semiconductor fabrication process employing the conventional KrF excimer laser and, as a result, ArF radiation is employed. When a photoresist for the TSI process using ArF is employed, it is possible to form the ultrafine pattern below 0.1 .mu.m L/S.
Recently, chemical amplification-type DUV photoresists have proven to be useful to achieve high sensitivity in processes for preparing micro-circuits in the manufacture of semiconductors. These photoresists are prepared by blending a photoacid generator with polymer (sometimes referred to herein as "resin") matrix macromolecules having acid labile structures, and the composition is coated on a substrate.
According to the reaction mechanism of such a photoresist, the photoacid generator generates acid when it is illuminated by a patterned light source, and the main chain or branched chain of the resin is reacted with the generated acid to be decomposed or crosslinked. The polarity change of the resin induces solubility differences between the exposed portion and unexposed portion in the developing solution, to form the predetermined pattern on the substrate.
In the lithography process, resolution depends upon the wavelength of the light source--the shorter the wavelength, the smaller the pattern that can be formed. However, when the wavelength of the light source is decreased in order to form a micro pattern, it is disadvantageous in that the lens of the exposing device is deformed by the light source, thereby shortening its life.
Melamine, a conventional cross-linker, has only three functional groups to cross-link with acid. Further, a large amount of acid must be generated when melamine is used as a cross-linker, because acid is consumed by the cross-linking reaction. As a result, higher energy light-exposure is required for such cross-linking agents. In order to overcome the disadvantages described above, chemical amplification-type components that cross-link with a photoresist resin and use less amounts of energy are desired.
Furthermore, developing solution may be soaked into the cross-linked site so as to swell it up. Thus, in order to form a pattern of higher integrity, the incorporation of a stronger cross-linker is required.
The photoresist composition must also have heat resistance so as to endure post exposure baking and the silylation process, which are performed at high temperature in the TSI process.
In order to overcome the above-described disadvantages, the present inventors have optimized the process for forming an ultrafine photoresist pattern for 4 G or 16 G DRAM semiconductor elements, by employing a photoresist composition comprising a chemical amplification-type cross-linker, which overcomes the problems of conventional cross-linkers, and a photoresist resin, which includes a polyvinyl hydroxyl group for heat resistance.